Rear projection screen

ABSTRACT

A screen for rear projection the image forming element which consists of a sheet having a thickness of 0.2 - 1.5 mm and consisting of opaline polymethyl methacrylate containing dispersed calcium and/or magnesium carbonate of a grain size less than 1 Mu , said sheet either having a covering at the rear side consisting of a transparent lacquer having a thickness of 10 100 Mu wherein glass particles having a grain size of 5 - 50 Mu are dispersed or glass particles of said kind being dispersed in the material of which the sheet consists.

United States Patent Clausen et al. [.4 1 Aug. 7, 1973 1 1 REARPROJECTION SCREEN 3,655,262 4/1972 De Palma 35(1/126 8 3O 8 D P'l I...35(l126 [76] Inventors: Erik Clausen; Johannes Clausen, 2 5 M972 6 ama at d 3 of DK'4540 a Primary Examiner-Samuel S. Matthews enmark,Assistant ExaminerRiehard A. Wintercorn [221 Flled? June 23,1972Attorney-Watson, Cole, Grindle & Watson [21] Appl. No.: 265,453 i I v[57] ABSTRACT [30] Forelg Appl'camm Prionty Data A screen for rearprojection the image forming element June 25, 1971 Denmark 3133/71 whichconsists of a sheet having a thickness of 0.2 1.5 mm and consisting ofopaline polymethyl methacrylate [52] US. Cl. 350/117, 350/126 containingdispersed calcium and/ormagnesium car- [51] Int. Cl. G03b 21/60 bonateof a grain size less than 1 [1,, said sheet either [58] Field of Search350/117, 126, 127, having a covering at the rear side consisting of atrans- 350/129; 161/3.5; 117/132 C; 264/1 parent lacquer having athickness of 10 100 wherein glass particles having a grain size of 5 50[L [56] I References Cited 1 are dispersed or glass particles of saidkind being dis- UNITED'STATES PATENTS persed in the material of whichthe sheet consists.

3,612,650 10/1971 Miyano et al 350/126 2 Claims, 3 Drawing Figures redblue-green minnow mm 7 red kin-green dag-green 1. REAR PROJECTION SCREENBACKGROUND OF THE INVENTION Rear projection screens are known in severalembodiments. E.g. such screens are known comprising a glass plate whichis made rough by etching or grinding and plastic plates treated in thesame way. Moreover, screens are known comprising glass or plastic platescovered by means of light diffusing coatings.

One problem to be solved by such screens is the avoiding of points ofscintillation. Such points may be avoided by using two screens eachprovided with a light diffusing surface or coating and by moving the twoscreens with respect to each other. Due to an interference phenomenonthe points are avoided in this way.

However, screens of the latter kind is rather difficult to use becausethey need a mutual movement and in case both screens are image forming,the image may be blurred due to the fact that an image is formed on bothscreens and in two different planes.

Accordingly, it is an object of the present inventio to provide a screenby means of which the image is formed in one plane only and whereinpoints of scintillation are avoided. According to the inventionithasbeenrealized that a plastic sheet consisting of an opaline or milkyplastic material is able to form an image without points 'ofscintillation but on the other hand such sheet has the drawback that thesource of light by means of which the image is projected will appearclearlyon the screen. Such appearance of the light source on aprojecting screen is termed hot-spot effect. However, according to thepresent invention it has been realized that such hot-spot effect maybeavoided by combining a plastic material as mentioned above with lightrefracting particles having a grain size between and 50 .4..

SUMMARY OF THE INVENTION A screen for rear projection comprising incombination an opaline plastic material and light refracting particleshaving a grain size between 5 and 50 a.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 illustrates the path of raysthrough a sheet of opaline plastic material,

FIG. 2 illustrates the path of rays through an embodiment of the rearprojection screen according to the invention, and

FIG. 3 the path of rays through another embodiment of the screenaccording to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT On the drawing, 1 is an opalineor milky plastic sheet consisting of polymethyl methacrylate or anothertransparent resin having corresponding index of refraction containing 1percent silicone oil of a viscosity of 100 c.p. and l o/oo benzoylperoxide wherein calcium and- /or magnesium carbonate having an averagegrain size less than 1 p. is dispersed in a concentration between 0.51.5 percent. Such opaline plastic material has a certain degree of lightconducting effect as indicated in FIG. 1 by means of an crumpled path ofrays. However, such path of rays apply only as regards rays from theshort wavelength range of the spectrum, i.e. the bluegreen range.However, more long waved light, i.e. light from the red range of thespectrum has a tendency to pass directly through the plastic material.Accordingly, screens for rear projection cannot be made from suchmaterial because the observer will be able to see the source of lightthrough the screen. According to the present invention it has beenrealized provided a material of the kind explained above contains lightrefracting particles having an index of refraction which is 5 15 percenthigher than the index of refraction of the plastic material and in asuitable small concentration, viz. between 1 15 percent so that theaverage spacing of the individual light refracting particles is lessthan the wavelength of long-waved light and of an average grain sizebetween 5 and 50 u as illustrated in FIG. 2, also the long-waved lightwill be refracted, viz. by means of the light refracting particles andwithout giving rise to any points of scintillation in the image formed.As light refracting particles glass particles are used, e.g. crown glassparticles which have an index of refraction which is 5 15 percent higherthan the index of refraction of polymethyl methacrylate.

Instead of dispersing the light refracting particles in the plasticmaterial for the manufacture of the screen as illustrated in FIG. 2,.thescreen according to the present invention may be constructed asillustrated in FIG. 3; In'FIG. 3, 1 is a sheet of opaline plasticmaterial of the same kind as explained with reference to FIG. 1. On therear side of the sheet I, i.e., the side on which the image isprojected, the wheel is provided with a coating consisting of atransparent lacquer wherein light refracting particles are dispersed,The lacquer does not contain more light refracting particles thannecessary for preventing long-waved light from passing the layer oflight refracting particles without being refracted. This effect isachieved in accordance with the present invention by selecting athickness of the layer of dispersed light refracting particles between10 and 10 p. and using an average grain size between 5 and 50 t. Also inthe embodiment illustrated in FIG. 3, light refracting particles areused, the index of refraction of which is between 5 and 15 percenthigher than the index of refraction of the lacquer (in the dry conditionthereof) wherein the particles are dispersed by coating the sheet 1. Inthe embodiment illustrated in FIG. 3,

also crown glass has been used for the light refracting particles andthe lacquer is a transparent thennoplastic lacquer sold in Denmark byBergs Lakfabrik" under the trademark Nr. 2000. However, also a lacquerwhich forms polymethyl methacrylate by the evaporation of the solventmay be used.

A sheet of polymethyl methacrylate having a thickness between 0.2 1 mmcontaining 1 percent finely pulverized calcium and/or magnesiumcarbonate, 1 percent silicone oil having a viscosity of c.p. and l o/oobenzoyl-peroxide has proved to have a suitable opaline degree. A higherconcentration of calcium and- /or magnesium carbonate up to 1.5 percentis used for sheets having a smaller thickness down to 0.2 mm and if athickness of the sheet up to 1.5 mm is selected a smaller concentrationdown to 0.5 percent calcium and/or magnesium carbonate is to be used.

It will be understood that FIGS. 2 and3 illustrate the image formingelement only for the screen according to the invention and if thiselement per se is not selfsupporting it is used in combination with aglass plate or a transparent plastic plate which serves as a carrier forthe screen element and supplies it with the strength required.

methyl methacrylate having a thickness between 0.2 mm and 1.5 mm andcontaining approximately 1 percent silicone oil having a viscosity ofapproximately 100 c.p. and approximately I oloo'benzoyl-peroxide wherein0.5 1.5 percent calcium and/or magnesium carbonate having an averagegrain size less than 1 p is dispersed, said sheet having a coatingconsisting of a transparent lacquer having a thickness of 100 p. andcontaining glass particles having an average grain size between 5 and 50u.

2. Rear projection screen comprising a sheet of polymethyl methacrylatehaving a thickness between 0.2 mm and 1.5 mm and containingapproximately 1 percent silicone oil having a viscosity of approximately100 c.p. and approximately 1 o/oo benzoyl-peroxide wherein 0.5 - 1.5percent calcium and/or magnesium carbonate having an average grain sizeless than 1 Mu is dispersed, said sheet having a coating consisting of atransparent lacquer having a thickness of 10 - 100 Mu and containingglass particles having an average grain size between 5 and 50 Mu .